1. Field of Invention
This invention relates generally to rudders for vessels, and more particularly to a sailboat rudder formed by a light weight core and a fiber-reinforced resin skin enveloping the core and conforming thereto to create a monocoque structure, and to a method for producing this rudder.
2. Status of Prior Art
A rudder is a mechanism for steering a ship or other vessel. A ship's rudder is formed by a blade supported on a stock which is hinged to the stern and controlled by a helm. When the ship is on a straight course, the blade of the rudder is then in line with the vessel. If the rudder is turned to one side or the other, the blade then offers sufficient resistance to the flow of water to deflect the stern and thereby change the direction of the ship. The structural strength of a rudder must be such that it is capable of withstanding the forces of water impinging on its surface, and its hydrodynamic design must be such that the rudder operates efficiently.
The concern of the present invention is with rudders for sailboats and the special requirements for such rudders, such as light weight and high strength, as well as a hydrodynamic design appropriate to the boat on which the rudder is installed.
A sailboat rudder now in widespread use has a stock formed of a stainless steel tube, rather than of a heavier solid metal. Welded onto the lower end of the stock are metal fins, the lower end of the stock being sandwiched between complementary half-sections of a blade. These half-sections are made of fiberglass-reinforced resin (FRP) skins which are held together by a foam-plastic core injected between the skins. Hence the blade itself is relatively light weight. But this known form of rudder, as will now be explained, suffers from several drawbacks.
A stainless steel tubular stock is subject to crevice corrosion and electrolysis, and it reacts differently to temperature changes than the half-sections of the FRP blade joined to the stock. As a consequence, the FRP blade sections will eventually sheer away from the steel stock, allowing water to penetrate the foam-plastic core through the exposed interface between the half-sections.
Another drawback is that when making this rudder, the FRP half-sections of the blade must be placed in a mold into which is injected the foam-plastic material which, when cured, forms the core that holds the blade half-sections together. With this molding method, one is unable to determine whether the foam-plastic core had cured evenly within the mold, yet only then does the core properly hold the blade half-sections together.
A more serious drawback of a prior art sailboat rudder of this type is that its hydrodynamic design is not determined by proper design considerations, but is dictated by the diameter of the tubular sheet stock inserted between the FRP complementary half-sections of the blade. Hence the blade may be unduly thick, making it easier to stall the boat and imparting poor lift characteristics thereto.
Still another drawback of this prior art sailboat rudder is that because the two FRP half-sections of the blade are held together by a foam-plastic core, the leading and trailing edges of the blade have an exposed interface. Hence should these edges of the blade be struck by floating debris or other objects, the two half-sections of the blade may then delaminate, with a resultant failure of the rudder.
In an aircraft, a monocoque structure is one in which the stressed outer metal skin carries all or a major portion of the torsional and bending stresses to which the structure is subjected. A rudder in accordance with the invention has a unique monocoque structure in that it is formed of a core whose shape and size define both the blade and stock of the rudder, the core being enveloped in an outer skin that conforms to the core and mainly bears the stresses to which the rudder is subjected.